My brother is 12 years younger than me and I coached his teams from the time he was 10 until the time he was 15. I also had a paying job while in college coaching a close by middle school team.
At all of those levels, pitches were called from the dugout and I had a very simple system that even a ten year old could understand. I wore a watch. I would flash a sign with each hand, the watch hand was live. After a little while, I would move the watch to the other hand. Nobody ever caught on and I got a lot of joy out of hearing the other dugout yell "curveball" shortly after a switched the watch to have a fastball go by the batter. After that, they are second guessing and the whole "stealing signs" things has been turned to your advantage.
BTW, I can't take credit for this simple sign system, it was used by one of my coaches when I was a kid. A little creativitiy goes a long way. It's not rocket surgery.
My dad was our third base coach and base running / batting coach when I was in high school
He was a master at signs
we did it by innings - even innings right hand right side of his body - odd innings left hand left side
I enjoyed the hell out of that
and he'd always announce the inning loudly
Yeah, there are many ways to do it.
So if a pitcher has three pitches, (1=fastball, 2=curve, 3=changeup), the catcher will flash 3 or 4 signs with a runner on second. The real sign can be either one of the four flashed signs and it is determined before each batter. I know a team that used colored cards that they would post in the dugout. Each color represented which sign was real (example: red for the first sign, white for the second, blue for the third, etc.). They even had decoy colors that defaulted to the first sign.
I've heard some teams go to a +1,-1 system where on a +1, a fastball (1) would be a 2 (1+1). A 3 would be a fastball as 3+1 would wrap around to the 1. The -1 would be the reverse. It's simple math but it seems that might cause some mistakes.